`impl Hash` for `SlotMap` and friends

[TheLostLambda]

It would be super helpful for storing structs containing a slotmap inside of things like quick-cache or similar!

[MaximoMachado]

In the meantime, as a workaround, you can impl Hash yourself for your struct since you can iterate over all of the items in slotmap and hash them manually. I just did something similar for PartialEq and Eq since slotmap does not do so.